Ultrasonic delay lines

ABSTRACT

Delay lines with a variety of delay versus frequency characteristics are obtained by fabricating the delay line apparatus from multilayer materials. In one modification, the delay line is of rod construction and in another, of strip design.

, United States Patent Armenakas 1 May 29, 1973 75 1 Inventor:

[54] ULTRASONIC DELAY LINES Anthony E. Armenakas, Beachhurst, N.Y.

[73] Assignee: The United States of America as represented by theSecretary of the Navy, Washington, D.C.

[22] Filed: July 14, 1971 [21] Appl. No-.: 162,583

[52] U.S. Cl. ..333/30 R, 333/71 [51] Int. Cl..... ..H03h 9/30 [58]Field of Search ..333/30 R, 30 M, 71, 333/72, 32

[56] I I References Cited UNITED STATES PATENTS r 3,264,583 8/1966 Fitch..333/30 R 2,549,578 4/1951 Curtis ..333/72 3,173,102 3/1965.Loewenstern, Jr.

3,464,033 8/1969 Tournois OTHER PUBLICATIONS Smith et al., DispersiveRayleigh Wave Delay Line Utilizing Gold on Lithium Niobate, MTT17,11-69,

Electronics, Microwave Acoustics Surfacing, Electronics 12-23-68, pp.95-96.

Matthews et al., Observation of Lone Wave Propagation at UHFFrequencies, Applied Physics Letters, Vol. 14, 1969, pp. 171-172. I

Daniel et al., Velocity Measurements of Elastic Surface Waves in theLayered System ZNS on A1 0 Applied Physics Letters, Vol. 16, 5-1-69, pp.

Tournois et 211., Use of Dispersive Delay Lines for Signal Processing inUnderwater Acoustics, Jr. Acoustical Soc. of Amen, Vol. 46, 1969, pp.517-531. J. deKlerk, Ultrasonic Transducers-Surface Wave TransducersUltrasonics, l-1971, pp. 35-48.

J. B. May, Jr., Wire-Type Dispersive Ultrasonic Delay Lines, IRE Trans.on Ultrasonic Eng, 6-1960, pp. 44-53.

C. C. Tseng, Elastic Surface Waves on Free Surface & Metallized Surfaceof Cds, ZNO & PZT-4, Jr. of

App. Physics, v01. 38, 1967, pp. 4281-4284.

Primary Examiner-Rudolph V. Rolinec Assistant Examiner-Wm. H. PunterAttorney- R. S. Sciascia and L. 1. Shrago [57] ABSTRACT Delay lines witha variety of delay versus frequency characteristics are obtained byfabricating the delay line apparatus from multilayer materials. in onemodification, the delay line is of rod construction and in another, ofstrip design.

3 Claims, 7 Drawing Figures TRANSDUCER TRANSDUCER Patented May'29, 19733 Sheets-Sheet 1 Patented May 29, 1973 MATERIAL l TRANSDUCER MATERIAL 2L m R E T A M 3 Sheets-Sheet 2 MATERIAL 3 Fig.3

TRANSDUCER TRANSDUCER TRANSDUCER Fig.4

/ MATERIAL I \-MATER|AL 2 K3? INTERDIGIT L INTERDIGITAL ELECTRODETRANSDUCER ELECTRODE TRANSDUCER e m r WA E V) o m n A Fig.6

Patented May 29, 1973 v 3,736,532

3 Sheets-Sheet :5

' u=2.0, p=|.o

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o 0 5 v.0 l5 2 O 52 Fig. 5

INTERDIGITAL ELECTRODE TRANSDUCER Fig.7

INVENTOR Anthony E. Armemzkos ULTRASONIC DELAY LINES The presentinvention relates generally to electroacoustic apparatus and, moreparticularly, to guided wave delay lines of the dispersive andnondispersive type wherein the delay versus frequency characteristic mayhave a range of possible slopes or curvatures.

In the conventional acoustic delay line, a piezoelectric transducertransforms an electrical signal into a mechanical deformation which thenpropagates as an elastic wave along a prescribed path through the delaymedium. In the usual case, such as the simple rod-type delay line, theelastic wave propagates essentially as a plane wave in an infinitemedium, free from any surface interactions.

In the guided wave acoustic delay line, the crosssectional'dimensionsof, 'for example, a wire or a rectangular strip are so chosen relativeto the wavelength that the elastic wave interacts strongly with theboundary surfaces and propagates as a guided elastic wave motion. Thus,there exists many possible modes of wave propagation and, in most ofthese modes, the phase velocity varies as a function of frequency. Inthis sense, the delay lines utilizing these modes are termed dispersive.

There are, however, exceptions to the above in the case of zeroth-ordermodes corresponding to thickness shear in a thin, rectangular strip andtorsional in a small diameter rod. These zeroth-order modesarenondispersive and, below the cut-off frequency of the lowest dispersivemode, they propagate as isolated modes of elastic wave motion. Theselast two modes thus can operate without an objectionable signaldistortion up to this cut-off frequency. Additionally, the low velocityof propagation of the torsional mode, as compared for example, to thefirst longitudinal mode, renders this mode most suitable fornondispersive delay lines where long delay times in the order ofmilliseconds are required.

One of the most important applications of a dispersive guided wave delayline with a linear delay characteristic is in radar systems forincreasing the range without necessitating a corresponding increase inpeak power. In the usual pulse radar system, range is increased byincreasing the average power radiated while range resolution isincreased by decreasing the pulse length. To increase range withoutcompromising resolution requires an increase in peak power which isultimately limited by voltage breakdown in the system. One solution tothis problem involves the pulse compression system which operates on thebasis that when a short pulse is transmitted to a linear delay networkof positive slope the various components of the Fourier frequencyspectrum of the pulse are linearly dispersed in time, the higherfrequencies being delayed more than the lower frequencies. The output isa linearly frequency modulated pulse with an amplitude distributiondescribed by the function sin x/x. Thereafter, this pulse, when it isreturned, for example, from a remote reflecting target, may betransmitted toa second delay network having an equal but negative slopeso that the components of the frequency spectrum will be delayed ininverted order, that is, the higher frequencies being delayed less thanthe lower frequencies. Alternatively, the pulse may be compressed byretransmission through the same delay line used for expanding theoriginal pulse, provided the order of the frequency is inverted bymodulating with a local oscillator frequency of twice the midbandfrequency of the input pulse and selecting the lower sideband of themodulation products. After such processing, it will be recognized, thefrequency components are restored to their initial phase relationshipand the output pulse will have the same shape as the input pulse.

In designing a dispersive delay line, made, for example, of a rod of agiven material, the thickness of the rod must be chosen so that theinflection point of the delay versus frequency curve occurs at a certainfrequency. The linearity and the slope of this curve may be altered bychanging the delaying material. But once the material is selected, thedelay characteristic may be altered only by changing the length of therod or by subdividing it into a series of lengths of differentthicknesses. Thus, dispersive delay lines made of a single material havesomewhat inflexible delay versus frequency characteristics.

It is accordingly a primary object of the present invention to provide adispersive delay line with a linear delay versus frequencycharacteristic whose slope may be readily selected within a range ofpossible values.

drical members;

It is another object of the present invention to provide a dispersivedelay line with a nonlinear delay versus frequency characteristic of adesired curvature.

Another object of the present invention is to provide a dispersive delayline operating in the zeroth torsional mode in rods or in the zerothface shear mode, in thin rectangular strips.

Another object of the present invention is to provide a nondispersivedelay line which is capable of delaying high frequency signals whereinthe acoustic signal is propagated as an interface disturbance.

Other objects, advantages and novel features of the inventionwill becomeapparent from the following detailed description of the invention whenconsidered in conjunction with the accompanying drawings wherein:

FIG. 1 is a family of curves showing the variations of the specificgroup delay with frequency for rods of different materials;

FIG. 2 is a plot of the same rods but with a different ratio of corethickness to casing thickness;

FIG. 3 illustrates one modification of the invention whereinthe delayline is composed of concentric, cylin- FIG. 4 shows an alternativeconstruction utilizing multilayered, rectangular strips;

FIG. 5 shows the variation of the specific group delay with frequency ofwaves traveling in the zeroth face shear mode in a structure like thatof FIG. 4;

FIG. 6 shows an alternative arrangement wherein the mechanicaldeformation is in the form of a Stoneley wave; and

FIG. 7 shows a delay line wherein the mechanical 'deformation is in theform of a Rayleigh wave.

Briefly and in general terms, the objects of the invention enumeratedabove are realized by fabricating the delay line apparatus frommultilayer materials instead of the'unitary material heretofore employedin prior art structures. With multilayer. materials, it will beappreciated, a broader choice of design parameters are available and agreater selection of delay versus frequency characteristics may beobtained. Thus, in one modification, the delay line consists of aplurality of concentric, cylindrical members made from differentmaterials. The geometrical and physical properties of the individualmembers may be selected to yield the performance curves desired. In analternative embodiment, a plurality of rectangular strips are utilizedto achieve the same flexibility. The first construction is utilized withwaves traveling in the zeroth torsional mode or in the firstaxisymrnetric, nontorsional mode. The second construction is utilizedwith waves propagating in the, zeroth face shear mode or in the firstlongitudinal mode.

Referring now to FIG. 1, the various curves shown illustrate thevariation of the specific group delay with frequency of waves travelingin the first axisymmetric, nontorsional mode in a composite rodconsisting of a circular core of one material bounded by and bonded to acircular casing of another material. It will be seen that all of thecurves, which represent different combinations of materials havingvarious density and stiffness 1 ratios, p and a, possess an inflectionpoint, such as points 1, 2 and 3 in FIG. 1, and that a linear range ofdelay versus frequency occurs over an operating range adjacent to thesepoints. The ratio of core thickness to casing thickness H is 4.5.

In FIG. 2 this ratio equals one-third, and it will be observed that thesame combinations of materialsnow yield a different set of delay versusfrequency curves. The inflection points are displaced from those of FIG.1, and the linear regions occur at different frequencies.

A guided delay line making use of the above characteristics is shown inFIG. 3. The apparatus consists of a solid inner rod or core member madeof a first material. A shorter, circular casing 11 of a second materialis bonded thereto, and still shorter length of circular casing 12 of athird material is bonded to this casing.

' Attached to opposite ends of the central core 10 are the piezoelectricinput and output transducers 13 and 14, respectively. As is well known,the orientation and construction of these transducers and the manner inwhich the input transducer is excited determine the particular modeexcited in the delay line.

Once the variation of delay versus frequency for a composite rodconsisting of a core member of one material, having a circular casing ofanother material bonded thereto, such as 10 and 11, has beenestablished, the addition of a third layer, such as 12, it will beappreciated, allows an added degree of freedom in the design of thedelay line. Changing the cross section of these elements, likewise,permits the designing of dispersive delay lines having a still widervariety of delay versus frequency characteristics. This may be explainedqualitatively by noting that this arrangement constitutes a seriescombination of cross sections and, consequently, the delay at frequencyf, of the rod assembly is given as U!) il l tZ Z ts a t2 4 u 5- where dd and d are the specific delays of the various portions of the rod, atthe frequency 12; L L L L L, are the lengths of the portions. The choiceof the materials 1, 2, 3 and the relative thick ness of the layersaffects the values of the specific delays (i (1, dig. However, the delayversus frequency characteristic will also depend upon the lengths L11LI? L3, L4, L5-

The individual components of the delay line may be made of any materialwhich is suitable for acoustic delay media, such as, for example,aluminum, nickeliron alloy, iron, fine grained bronze or any other finegrained material.

It has been determined by mathematical analysis that the zerothtorsional mode in a multilayer rod arrangement, such as shown in FIG. 3,is dispersive. This same mode in a unitary rod, it will be recalled, isnondispersive. Consequently, this zeroth torsional mode with itsadvantages of low signal distortion and low velocity of propagation maybe utilized to provide a new class of dispersive guided wave delaylines.

In FIG. 4 there is disclosed an analogous multilayer guided wave delayline fashioned from a plurality of relatively thin, rectangular stripsof different metal. Here, the piezoelectric input and output transducers20 and 21 are secured to opposite end faces of an inner rectangularstrip 22. Bonded to its opposite surfaces are a first pair of shorterrectangular strips 23 and 24. A second pair of still shorter strips 25and 26 are bonded to these strips. Each pair of strips is made of thesame material so that the over-all stepped sandwich has a symmetricalconfiguration and composition.

It has also been determined mathematically that the zeroth face shearmode in a multilayer plate assembly, such as shown in FIG. 4, isdispersive. This, too, is in contradistinction with the same modepropagating in a unitary plate of a single material and may be utilizedto provide a new class of dispersive guided wave delay lines.

It should be appreciated that the consecutive casings, where the delayline is of a rod design or the consecutive strips where the line is madeof such strips, need not be of shorter length such as depicted in FIGS.3 and 4. What is important is that the cross section of the delay mediumchanges along its length. Itwill be appreciated that the length ofproportions L,, L L L L and the materials from which these componentsare made will be selected in order to achieve the desired performancecurve.

FIG. 5 illustrates the variation of the specific group delay withfrequency of waves traveling in the zeroth face-shear mode in athree-layer plate construction of the type shown in FIG. 4.

The arrangements as shown in FIGS. 3 and 4 are capable of linearlydelaying pulses having a considerably larger bandwidth than has beenpossible heretofore. This improvement is due to the more extensivelinearity of their delay versus frequency characteristics, asexemplified by the curves of FIG. 5.

The frequencies at which nondispersive delay lines operate withoutobjectionable signal distortion do not exceed a few megacycles. In FIG.6 there is disclosed a composite delay line which is capable ofoperating at considerably higher frequencies. The apparatus consists ofa two-layer, rectangular plate made by bonding together two differentstrips, 30 and 31, of similar dimensions. The composite plate is drivenby an interdigital electrode transducer 32 which excites a Stoneley wavewhich propagates as an interface disturbance. The output signal isremoved by a second interdigital electrode transducer 33. It will beappreciated that the individual comblike strips that form eachtransducer are provided with suitable insulating coatings to protectagainst shorting by the confronting surfaces of the two strips. One ofthe materials that lends itself to this type of delay line is siliconwhich exhibits relatively low losses in the microwave signal region.

FIG. 7 shows an arrangement in which the the signal is propagated as aninner surface Rayleigh wave in a hollow cylinder made of two concentricshells 40 and 41. One of the conditions for operation is that the shearvelocity in the material of the inner cylinder be smaller than in thematerial of the outer layer. The Rayleigh waves are produced by aninterdigital electrode transducer 42 affixed to the inner wall surfaceof the inner shell of the hollow composite cylinder. The output signalis extracted by a similar transducer located at the other end of thecylinder.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed is:

1. A solid delay line comprising, in combination,

a first cylindrical shell member made of a material that is capable ofpropagating elastic waves;

an input interdigital electrode transducer secured to the inner wallsurface of said shell member at a position adjacent one end thereof;

an output interdigital electrode transducer secured to the inner wallsurface of said shell member at a location adjacentthe other endthereof;

said transducers being in alignment such that an elastic wave excited bysaid input transducer is subsequently detected by said outputtransducer;

a second cylindrical shell member made of a different material that isalso capable of propagating elastic waves, the inner surface of saidsecond cylindrical shell member being bonded to the outer surface ofsaid first inner cylindrical shell member over their common length, thethickness of said first cylindrical shell member being such that saidelastic wave also travels through portions of said second cylindricalshell member. 2. A delay line comprising in combination a solid rod madeof a first material that is capable of propagating elastic waves;

an input transducer secured to one end of said rod;

an output transducer secured to the other end of said rod;

a first cylindrical shell member having a length less than that of saidrod bonded to the outer surface of said rod, said shell member beingmade of a second material that is also capable of propagating elasticwaves;

a second cylindrical shell member having a length less than that of saidfirst cylindrical shell member, said second cylindrical shell memberbeing bonded to the outer surface of said first cylindrical shell memberand being made of a material that is capable of propagating elasticwaves,

The lengths of said rod, and said first and second cylindrical shellmembers and the density and stiffness ratios of said rod and said firstand second cylindrical shell members being selected to achieve. thesignal time delay desired.

3. A solid delay line comprising in combination a pair of unequal lengthstrips made of different metals that are capable of propagating elasticwaves; said strips being bonded together over their common length andhaving a thickness such that any elastic wave excited in the longerstrip travels also within the shorter strip when it reaches one end ofthis strip;

an input transducer secured to one end face of the longer strip;

an output transducer secured to the other end face of said longer strip;

a third strip having a length less than the longer strip of said pairand bonded to said longer strip over its length,

said elastic wave also traveling within said third' strip when itreaches one end thereof; the lengths of said strips and their densityand stiffness ratios being selected to obtain the desired signal timedelay.

1. A solid delay line comprising, in combination, a first cylindricalshell member made of a material that is capable of propagating elasticwaves; an input interdigital electrode transducer secured to the innerwall surface of said shell member at a position adjacent one endthereof; an output interdigital electrode transducer secured to theinner wall surface of said shell member at a location adjacent the otherend thereof; said transducers being in alignment such that an elasticwave excited by said input transducer is subsequently detected by saidoutput transducer; a second cylindrical shell member made of a differentmaterial that is also capable of propagating elastic waves, the innersurface of said second cylindrical shell member being bonded to theouter surface of said first inner cylindrical shell member over theircommon length, the thickness of said first cylindrical shell memberbeing such that said elastic wave also travels through portions of saidsecond cylindrical shell member.
 2. A delay line comprising incombination a solid rod made of a first material that is capable ofpropagating elastic waves; an input transducer secured to one end ofsaid rod; an output transducer secured to the other end of said rod; afirst cylindrical shell member having a length less than that of saidrod bonded to the outer surface of said rod, said shell member beingmade of a second material that is also capable of propagating elasticwaves; a second cylindrical shell member having a length less than thatof said first cylindrical shell member, said second cylindrical shellmember being bonded to the outer surface of said first cylindrical shellmember and being made of a material that is capable of propagatingelastic waves, The lengths of said rod, and said first and secondcylindrical shell members and the density and stiffness ratios of saidrod and said first and second cylindrical shell members being selectedto achieve the signal time delay desired.
 3. A solid delay linecomprising in combination a pair of unequal length strips made ofdifferent metals that are capable of propagating elastic waves; saidstrips being bonded together over their common length and having athickness such that any elastic wave excited in the longer strip travelsalso within the shorter strip when it reaches one end of this strip; aninput transducer secured to one end face of the longer strip; an outputtransducer secured to the other end face of said longer strip; a thirdstrip having a length less than the longer strip of said pair and bondedto said longer strip over its length, said elastic wave also travelingwithin said third strip when it reaches one end thereof; the lengths ofsaid strips and their denSity and stiffness ratios being selected toobtain the desired signal time delay.